Recent changes in safety requirements in the aerospace industry have meant that more and more systems, subsystems and pieces of equipment form part of the Functional Hazard Analysis (FHA). The FHA identifies system failures and identifies the effects of these failures. Failures are tabulated and classified according the effects which that failure may cause and the safety objectives assigned according to the criteria are shown below:
Failure conditionDevelopmentSafety objectives quantities requiredclassificationassurance level(probability per flight hour)CatastrophicA<1 × 10−9Hazard/SevereB<1 × 10−7MajorC<1 × 10−5MinorD<1 × 10−3No safety effectENone
In essence, failure conditions may result in a piece of equipment failing to do something, or inadvertently doing something it shouldn't. It may also comprise a failure to indicate an event or a failure to indicate that an event has not happened.
One such subsystem required to fulfil the FHA is landing gear detectors (for example proximity sensors), which are located on the fuselage or in the landing gear, and interacts with the fuselage or landing gear to determine whether or not the landing gear has been deployed or retracted. Clearly the pilot would wish to know with confidence whether or not the landing gear has been deployed when attempting to land so that he can chose the most appropriate course of action should there be a problem with the landing gear deploying.
In order to fulfil the FHA, one solution is to have two independent landing gear detectors located to determine the location of the landing gear. In the event that one detector fails, the secondary detector can still give a reliable reading. However, this is not always possible due to weight and space constraints in locating the secondary detector. In practice, these landing gear detectors are proximity sensors, which detect changes in the impedance of a component depending on the location of a target.
Since there are already proximity sensors in landing gear components performing the task of detecting the location of the landing gear, we have therefore appreciated the need for a proximity sensor monitor for monitoring the status of a proximity sensor (for example with use with landing gear), and improved proximity sensor systems incorporating the proximity sensor monitor.